Chandra Shekhar Pandey

The relaxor behavior of tetragonal tungsten bronze uniaxial relaxor ferroelectric calcium strontium barium niobate (Ca_0.22Sr_0.12Ba_0.66Nb₂O₆ or CSBN-22) single crystal was studied by measuring elastic constants and thermal expansion with the aid of resonant ultrasound spectroscopy and dilatometry, respectively, in the temperature range 300 K–1503 K. Thermal expansion yields evidence of the Burns temperature T_B and the intermediate characteristic temperature T*, which was also supported by the temperature evolutions of the elastic constants c_ij. CSBN-22 was found to be ∼2%–3% elastically stiffer than CBN-28. The presented results open the perspective to understand the relaxor behavior of CSBN.

The incommensurately modulated crystal structures of Ca_0.28Ba_0.72Nb₂O₆ (CBN28) and Ce_0.02Ca_0.25Ba_0.72Nb₂O₆ (Ce:CBN28) were refined in the supercentred setting X4bm(AA0,−AA0) of the 3 + 2-dimensional superspace group P4bm(aa½,−aa½). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO₆ octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium.

The full sets of elastic constants (considering piezoelectric effect, i.e., taking into account piezoelectric coupling effects) and piezoelectric stress constants of five natural tourmaline single crystals of different chemical composition have been determined from room temperature up to ∼900 K employing resonant ultrasound spectroscopy. Further, the coefficients of thermal expansion were studied in a wide temperature range from 100 K to 950 K. Electron microprobe analysis showed that the chemical composition of investigated samples varied over a wide range, which allows to study the compositional dependence of thermal, elastic, and piezoelectric properties of tourmalines. At room temperature, the piezoelectric constants of tourmalines were found at-least 1.5 times higher than that for α-quartz. Importantly, we find that the application of tourmaline at temperatures up to its decomposition temperature is not limited by phase transitionsor any ultrasound dissipation effects.

Elastic behavior of tetragonal tungsten bronze uniaxial relaxor ferroelectric cerium doped Ca_0.28Ba_0.72Nb₂O₆ single crystal was investigated employing resonant ultrasound spectroscopy in the temperature range from room temperature up to 1323 K. Doping of cerium lowers the phase transition temperature T_c and Burns temperature T_b significantly, however, intermediate characteristic temperature T* (between the Burns temperature T_b and the temperature of maximum dielectric permittivity T_m) remains same as for pure Ca_0.28Ba_0.72Nb₂O₆. All independent elastic constants evolved differently with temperature, reflecting their coupling to different types of the reorientational motion of the polar nanoregions through their interaction with the acoustic waves.

Full sets of elastic constants cij of tetragonal tungsten bronze relaxor ferroelectric Ca0.28Ba0.72Nb2O6 (CBN-28) single crystals are measured above Curie temperature up to 1503~K employing resonant ultrasound spectroscopy. Thermal expansion measurements on as-grown unpoled CBN-28 reveal the existence of a characteristic temperature T\,* ($\sim${}800~K) for CBN-28 between the Burns temperature Tb ($\sim${}1100~K) and the temperature of maximum dielectric permittivity Tm ($\sim${}600~K). The influence of polar nanoregions (PNRs) on the elastic properties of CBN-28 is studied in detail. The temperature evolution of cij shows pronounced anomalies. All independent elastic constants evolved differently, with temperature reflecting their coupling to different types of the reorientational motion of PNRs through their interaction with acoustic waves. The anisotropy of longitudinal elastic stiffness coefficients and the deviation from Cauchy relations for CBN-28 are also studied, showing the evolution of material anisotropy and the nature of bonding interactions with temperature, respectively.

Dielectric responses were studied on piezoelectric tourmaline single crystals of widely varying chemical composition from different geological origins. The dielectric constants at constants stress, and dissipation factor were measured as a function of frequency (100-1000 kHz) using method of substitution. A correlation between two independent dielectric constants (along and perpendicular to crystallographic c-axis) is observed, and dependence of dielectric constants on chemical composition is presented.